Wireless communications use various forms of signals, such as radio frequency (RF) signals, to transmit data. As a signal travels from a broadcasting transmitter, the signal loses power or attenuates. The signal also encounters various forms of interference along the signal path which introduce noise into the signal. A transmitter that is in relatively close proximity to a receiver can introduce significant noise into the signal being received (i.e., co-site interference). Devices which include both a transmitter and a receiver typically attempt to isolate the transmitter from the receiver so that the co-site interference does not overpower the receiver.
Most RF communication systems utilize frequency-division duplexing (FDD) where the uplink and downlink are on different RF spectral bands, in order to simplify the co-site interference problem. For example, the maximum power for a typical base station transmission is 30 dBm, while the receiver sensitivity or minimum required receive power is −100 dBm. Therefore, the required isolation can be as high as 130 dB. Traditionally, bandpass filters are used to isolate the high power co-site interference from the receive path to allow for reception. If a traditional receiver were operating in the spectrum 707-710 MHz, a bandpass filter would be used to remove all frequencies outside of the 707-710 MHz band. However, this is not possible if a single link is processing multiple bands. For example, in the case of the approximately 20 existing and planned LTE bands implemented over the spectrum between 455-2140 MHz, the high power transmissions are interspersed over the entire spectrum. Approaches more efficient than simply using a bandpass filter are needed for handling the high power level associated with these types of co-site transmissions, which can be in the same band or in different bands.